Typically, an engine cooling system comprises a primary cooling circuit having a pump to circulate coolant used to cool the engine and a radiator to cool the coolant. However, coolant heated by the engine can be used for heating purposes, typically a passenger compartment heater for occupants of a vehicle. Also, the engine coolant may be used in a secondary cooling circuit to cool one or more auxiliary heat exchangers such as a transmission oil cooler, an engine oil cooler, a fuel cooler or an air-charge cooler.
It can be advantageous, particularly in the case of a transmission oil cooler, if the coolant circulating through the cooling system can be used to provide heat to the auxiliary heat exchanger. In the case of the transmission oil cooler, this warms the transmission directly after engine start up so as to enable the transmission to reach its maximum operating efficiency as soon as possible. When the transmission has warmed up the coolant can then be used to prevent overheating of the transmission oil, particularly when the transmission is under heavy load. However upon start up from cold, it is also desirable to ensure that any passenger compartment heater which uses the engine coolant as a source of heat is heated as soon as possible. It is therefore desirable to provide some means of ensuring prompt passenger compartment warm up while utilizing any spare heat from the engine for an auxiliary heat exchanger.
It is an object of this invention to provide an engine cooling system that is able to maximize its heating performance in the period after start up from ambient while also using excess heat to warm a secondary coolant flowing to an auxiliary heat exchanger.